Fix memory leak for incomplete lambdas

[Bug #19836]

The parser does not free the chain of `struct vtable`, which causes
memory leaks.

The following script reproduces this issue:

```
10.times do
  100_000.times do
    Ripper.parse("-> {")
  end

  puts `ps -o rss= -p #{$$}`
end
```

1 files changed, 14 insertions, 10 deletions

diff --git a/parse.y b/parse.y
index 0909356f33..c1c8a7514b 100644
--- a/parse.y
+++ b/parse.y
@@ -13229,22 +13229,26 @@ local_push(struct parser_params *p, int toplevel_scope)
 }
 
 static void
-local_free(struct parser_params *p, struct local_vars *local)
+vtable_chain_free(struct parser_params *p, struct vtable *table)
 {
-    if (local->used) {
-        vtable_free(local->used);
+    while (!DVARS_TERMINAL_P(table)) {
+        struct vtable *cur_table = table;
+        table = cur_table->prev;
+        vtable_free(cur_table);
     }
+}
+
+static void
+local_free(struct parser_params *p, struct local_vars *local)
+{
+    vtable_chain_free(p, local->used);
 
 # if WARN_PAST_SCOPE
-    while (local->past) {
-        struct vtable *past = local->past;
-        local->past = past->prev;
-        vtable_free(past);
-    }
+    vtable_chain_free(p, local->past);
 # endif
 
-    vtable_free(local->args);
-    vtable_free(local->vars);
+    vtable_chain_free(p, local->args);
+    vtable_chain_free(p, local->vars);
 
     ruby_sized_xfree(local, sizeof(struct local_vars));
 }